Many indwelling medical devices have a hollow portion. For example, stents are hollow devices that are inserted into body ducts for preventing narrowing of the duct lumen, for tutoring a dilated lumen or for acting as a substrate for tissue growth. As another example, a catheter may have a hollow portion that may serve to transfer a fluid from outside the body to a body cavity, or for draining fluid from a body cavity. As yet another example, an artificial blood vessel valve has a casing enclosing a space through which blood flows.
The hollow portion of a medical device may have a fixed caliber in which it is both delivered and deployed. Alternatively, the hollow portion may be brought into an initial, small caliber, conformation in which it is inserted into the body and delivered to the site where it is to be deployed. This allows the hollow portion to be delivered with minimal damage to surrounding tissues. Deployment of the device involves expanding the hollow portion to a final larger caliber. When it is desired to remove the device from the body, the hollow portion may first be made to return to the small caliber conformation and then removed. For example, U.S. Pat. No. 5,037,427 discloses a stent made from a two-way shape memory alloy. This stent has a transition temperature that is below body temperature in which it changes its diameter from a narrow diameter to a wide diameter. The stent is inserted into the body under a constant flow of cold fluid in order to maintain the stent in the narrow diameter during delivery. Once in the stent has been positioned in the desired location, the flow of the cold fluid is stopped and the stent then expands either spontaneously as it warms up to body temperature or by flowing a warm fluid around the stent. When the stent is to be removed, a flow of cold fluid is again applied to the stent causing the stent to soften and return to the narrow diameter conformation. The flow of cold fluid is maintained until the stent is removed from the body.